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July 4th, 2009

Strides made in nanotube-based mass measurements

Published on July 4th, 2009 | By: pwray@ceramics.org

According to the ICT Results website, researchers at a European Union-sponsored project say their method to use carbon nanotubes to measure masses at the atomic level is improving.

European researchers claim their device, one that uses carbon nanotubes and already shown to be adept at measuring the mass of some larger atoms, is getting better at measuring smaller atoms, and should ultimately be used to measure the mass of protons and neutrons. It may also help scientists study the progress of a chemical reaction.

This work is being carried out under the “CARDEQ” project, a collaboration among seven European universities.

According to the ICT Results website, the mass-measurement method relies on the behavior of heavier nanotubes vibrating slower than lighter ones. The research group turns a single nanotube into a transistor (the semiconductor properties of nanotubes can be manipulated by how a graphene sheet is wound). They then use the nanotube’s vibration to modulate a current passing through it.

“The suspended nanotube is, at the same time, the vibrating element and the readout element of the transistor,” says Pertti Hakonen, a professor at the Helsinki University of Technology.

Hakonen says the group has settled in on two different approaches to using this transistor effect. “We have the single electron transfer concept, which is more sensitive, and the field effect transistor concept, which is faster,” he says. “The most straightforward is that you map out the response curve then detect the frequency shift. The other way is you vibrate the string at a fixed frequency and see how the response changes. When the frequency moves, then the response fixed frequency will change.”

CARDEQ demonstrated the ability to measure the mass of chromium atoms, which are evaporated onto the nanotube, last year.

The scientists built a semiconducting nanotube into a transistor so that the vibration modulates the current passing through it, making a single nanotube both the vibrating element and the readout element. To get a single atom on the nanotube, metal is evaporated and by chance one will impinge on the string.

Hakonen acknowledges that similar work is going on at University of California, Berkeley and Caltech, but says that his group is getting more accurate results.

Hakonen says the method can now measure atoms the size of argon, but stability is still a problem. “When the device is operating well, we can see a single argon atom on short time scales. But then if you measure too long the noise becomes large,” he says.

Still, he thinks the methodology will ultimately measure the mass of a proton or neutron. “It’s a big difference,” he admits, “but typically the improvements in these devices are jump-like. It’s not like developing some well-known device where we have only small improvements from time to time. This is really front-line work and breakthroughs do occur occasionally.”

The hope is that they will eventually be able to make mass measurements of different types of molecules and atoms in real time. This would, for example, provide insights on chemical-biological processes, or even radioactive decay and other quantum phenomena.

But, practical applications aren’t necessarily right around the corner. “It will depend very much on how the technology for processing carbon nanotubes develops. I cannot predict what will happen, but I think chemical reactions in various systems, such as proteins and so on, will be the main applications in the future,” predicts Hakonen.

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